We have succeeded in the preparation of three kinds of "green hemoglobin" by chemical modifications of the prosthetic groups of human hemoglobin. The modifications were made by the oxidation of protohemin with potassium permanganate. The vinyl groups of the hemin were oxidized to formyl groups and three kinds of formyl derivatives were formed depending on the number and the position of the formylation. These three formyl derivatives of hemin were recombined with human apohemaglobin to form green hemoglobins. One of the three derivatives (2-formyl-4-vinyl hemoglobin) has oxygen binding properties fairly close to normal hemoglobin. Therefore, this green hemoglobin will be used as a marker by mixing with normal red hemoglobin. In addition, the detailed biophysical studies of these green hemoglobins will provide important information about the mechanism of oxygen binding to hemoglobin with respect to the structure of the prosthetic groups.